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From: Jeff Higgins <jeff@invalid.invalid>
Newsgroups: comp.lang.c,comp.lang.java.programmer
Subject: Re: Self-updating relationship
Date: Wed, 02 Sep 2015 23:38:54 -0400
Organization: A noiseless patient Spider
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References: <relationship-20150902211808@ram.dialup.fu-berlin.de> <55E766BD.3060303@verizon.net> <spaces-20150902222406@ram.dialup.fu-berlin.de> <55E77D04.6080200@verizon.net>
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On 09/02/2015 06:49 PM, James Kuyper wrote:
> On 09/02/2015 05:25 PM, Stefan Ram wrote:
>> James Kuyper <jameskuyper@verizon.net> writes:
>>>> final javafx.beans.property.IntegerProperty height
>>>> = new javafx.beans.property.SimpleIntegerProperty( 10 );
>>>> final javafx.beans.property.IntegerProperty number
>>>> = new javafx.beans.property.SimpleIntegerProperty(  2 );
>>>> final javafx.beans.binding.NumberBinding spaces =
>>>> height.subtract( number );
>>>> java.lang.System.out.println( spaces.getValue() );
>>>> height.set( 20 );
>>>> java.lang.System.out.println( spaces.getValue() ); }}
>>> What's the java feature that makes "spaces" above update automatically?
>>> To put it another way - if I didn't want such updates, what should I
>>> write differently?
>>
>>    This depends on the details of the behavior you want instead.
>
> I read a bit about Java many years ago, and reached the conclusion that
> it had some syntactic similarities C++, but had a very different way of
> implementing objects. I played with it long enough to get something
> working, but concluded that it wasn't what I needed for any purpose I
> was trying to achieve at the time. I'm otherwise very unfamiliar with java.
>
> I get the impression from what you're saying, that the code you've shown
> results in each call to spaces.getValue() performing a fresh calculation
> of height-number, using the values of those things at the time
> getValue() is called. I'm asking about what you would have to do
> differently to have spaces set to the value of height - number at one
> particular time, and to have spaces.getValue() continue to report that
> same value, even if the values of height and number later change. It
> seems to me that this should be easier than arranging for automatic
> updates - in fact, I'd expect it to be trivial.
>
>>    For one example, you can simply omit »height.set( 20 )«, and
>>    »spaces« will not be updated. Or, you could just write:
>
> Well, I'm asking how to ensure that "spaces" will not be updated, even
> if "height" is, so that doesn't qualify.
>
>> public final class Main
>> { public static void main( final java.lang.String[] args )
>>    { final javafx.beans.property.IntegerProperty spaces
>>      = new javafx.beans.property.SimpleIntegerProperty( 8 );
>>      java.lang.System.out.println( spaces.getValue() ); }}
>>
>>    Which just will print »8« and not update spaces, or just
>>
>> public final class Main
>> { public static void main( final java.lang.String[] args )
>>    { int const spaces = 8; java.lang.System.out.println( spaces ); }}
>
> Those both set spaces to 8, rather than height - number. If
> height-number is currently 8, that happens to be the right value, but
> isn't what I'm looking for.
>
>>    or just,
>>
>> public final class Main
>> { public static void main( final java.lang.String[] args )
>>    { java.lang.System.out.println( 8 ); }}
>>
>>    to get the same behavior.
>
> That seems to print out 8 without even storing it anywhere, and without
> any connection to "height" or "number".
>

You may use the unbind method of the
javafx.beans.property.Property<T> interface.
<https://docs.oracle.com/javase/8/javafx/api/javafx/beans/property/Property.html>

import javafx.beans.property.IntegerProperty;
import javafx.beans.property.SimpleIntegerProperty;

public class Main
{ public static void main(String[] args)
   {

     final IntegerProperty height
     = new SimpleIntegerProperty();

     final IntegerProperty number
     = new SimpleIntegerProperty();

     final IntegerProperty spaces
     = new SimpleIntegerProperty();

     spaces.bind(height.subtract(number));

     height.set(10);
     number.set(2);
     System.out.print(spaces.get()); // 8

     spaces.unbind();
     height.set(20);
     System.out.print(spaces.get()); // 8

} }